Magnetoelectronic devices, spin electronic devices, and spintronic devices are synonymous terms for devices that make use of effects predominantly caused by electron spin. Magnetoelectronics are used in numerous information devices, and provides non-volatile, reliable, radiation resistant, and high-density data storage and retrieval. The numerous magnetoelectronics information devices include, but are not limited to, Magnetoresistive Random Access Memory (MRAM), magnetic sensors, and read/write heads for disk drives.
Typically, a magnetoelectronic information device, such as an MRAM, includes an array of memory elements. Each memory element typically has a structure that includes multiple magnetic layers separated by various non-magnetic layers. Information is stored as directions of magnetization vectors in the magnetic layers. Magnetic vectors in one magnetic layer are magnetically fixed or pinned, while the magnetization direction of another magnetic layer may be free to switch between the same and opposite directions that are called “parallel” and “antiparallel” states, respectively. Corresponding to the parallel and antiparallel magnetic states, the magnetic memory element has low and high electrical resistance states, respectively. Accordingly, a detection of the resistance allows a magnetoelectronics information device, such as an MRAM device, to provide information stored in the magnetic memory element.
MRAM technology uses magnetic components to achieve non-volatility, high-speed operation, and excellent read/write endurance. FIG. 1 illustrates a conventional memory element array 10 having one or more memory elements 12. An example of one type of magnetic memory element, a magnetic tunnel junction (MTJ) element, comprises a fixed ferromagnetic layer 14 that has a magnetization direction fixed with respect to an external magnetic field and a free ferromagnetic layer 16 that has a magnetization direction that is free to rotate with the external magnetic field. The fixed layer and free layer are separated by an insulating tunnel barrier layer 18. The resistance of memory element 12 relies upon the phenomenon of spin-polarized electron tunneling through the tunnel barrier layer between the free and fixed ferromagnetic layers. The tunneling phenomenon is electron spin dependent, making the electrical response of the MTJ element a function of the relative magnetization orientations and spin polarization of the conduction electrons between the free and fixed ferromagnetic layer.
The memory element array 10 includes conductors 20, also referred to as digit lines 20, extending along rows of memory elements 12, conductors 22, also referred to as word or bit lines 22, extending along columns of the memory elements 12, and conductor 19, also referred to as an electrode 19, electrically contacting the fixed layer 14. While the electrodes 19 contact the fixed ferromagnetic layer 14, the digit line 20 is spaced from the electrodes 19 by, for example, a dielectric material (not shown). A memory element 12 is located at a cross point of a digit line 20 and a bit line 22. The magnetization direction of the free layer 16 of a memory element 12 is switched by supplying currents to electrode 19 and bit line 22. The currents create magnetic fields that switch the magnetization orientation of the selected memory element from parallel to anti-parallel, or vice versa. To sense the resistance of element 12 during the read operation, a current is passed from a transistor in the substrate (not shown) through a conductive via (not shown) connected to electrode 19.
FIG. 2 illustrates the fields generated by a conventional linear digit line 20 and bit line 22. To simplify the description of MRAM device 10, all directions will be referenced to an x- and y-coordinate system 50 as shown. A bit current IB 30 is defined as being positive if flowing in a positive x-direction and a digit current ID 34 is defined as being positive if flowing in a positive y-direction. A positive bit current IB 30 passing through bit line 22 results in a circumferential bit magnetic field, HB 32, and a positive digit current ID 34 will induce a circumferential digit magnetic field HD 36. The magnetic fields HB 32 and HD 36 combine to switch the magnetic orientation of the memory element 12.
The traditional MRAM switching technique, using magnetic fields generated by current-carrying lines adjacent to the memory element, depicted in FIG. 2, has some practical limitations, particularly when the design calls for scaling the bit cell to smaller dimensions. For example, since this technique requires two sets of magnetic field write lines, the array of MRAM cells is susceptible to bit disturbs, i.e., neighboring cells may be unintentionally altered in response to the write current directed to a given cell. In some embodiments of field-switched MRAM, the elements not located at the cross point of the two active write lines experience only one of HD 36 and HB 32, known as half-selected elements, have a reduced energy barrier to reversal and therefore have a significant probability of unintentional reversal. Furthermore, decreasing the physical size of the MRAM elements results in lower magnetic stability against magnetization switching due to thermal fluctuations, because the energy barrier to thermal reversal decreases with decreasing free layer volume. The stability of the bit can be enhanced by utilizing a magnetic material for the free layer with a large magnetic anisotropy and therefore a large switching field, but then the currents required to generate a magnetic field strong enough to switch the bit can be impractical in real applications.
In spin-torque MRAM (ST-MRAM) devices, such as the structure 100 shown in FIG. 3, the bits are written by forcing a current 40 directly through the stack of materials that make up the magnetic tunnel junction 12, e.g., via current controlled via isolation transistor 42. Generally speaking, the write current 40 which is spin polarized by passing through one ferromagnetic layer (14 or 16), exerts a spin torque on the subsequent layer. This torque can be used to switch the magnetization of free magnet 16 between two stable states by changing the write current polarity.
The spin-torque effect is known to those skilled in the art. Briefly, a current becomes spin-polarized after the electrons pass through the first magnetic layer in a magnet/non-magnet/magnet trilayer structure, where the first magnetic layer is substantially fixed in its magnetic orientation by any one of a number of methods known in the art. The spin-polarized electrons cross the nonmagnetic spacer and then, through conservation of spin angular momentum, exert a spin torque on the second magnetic layer, which switches the magnetic orientation of the second layer to be parallel to the magnetic orientation of the first layer. If a current of the opposite polarity is applied, the electrons instead pass first through the second magnetic layer. After crossing the nonmagnetic spacer, a spin torque is applied to the first magnetic layer. However, since its magnetization is fixed, the first magnetic layer does not switch. Simultaneously, a fraction of the electrons will then reflect off the first magnetic layer and travel back across the nonmagnetic spacer before interacting with the second magnetic layer. In this case, the spin torque acts so as to switch the magnetic orientation of the second layer to be anti-parallel to the magnetic orientation of the first layer. Spin-torque switching occurs only when the current 40 exceeds the critical current IC of the element. The spin-torque switching current used by the circuit is chosen to be somewhat above the average IC of the memory elements so that all elements will switch reliably when the switching current is applied.
However, these known ST-MRAM cells require a high current density to switch the direction of the free layer. While an easy axis assist field could be used to reduce the switching current density, this field also reduces the energy barrier of the half-selected elements in an array for magnetization reversal due to thermal fluctuations.
Accordingly, it is desirable to provide a magnetoelectronic information device and switching method that reduces the required spin-torque current density and cell size without unwanted reversal of non-selected elements in the array. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.